1. Field of the Invention
The present invention relates generally to seismic exploration and, more specifically, to a method, apparatus and system for use in detecting waves in an encased borehole.
2. State of the Art
Seismic surveying is used to examine subterranean geological formations for the potential presence of hydrocarbons such as oil, natural gas and combinations thereof as well as the extent or volume of such reserves. Seismic waves are emitted from a seismic source to penetrate through layers of rock and earth, and under certain conditions the waves are reflected and refracted by variations in the composition of the subterranean formations. Seismic sensors, such as, for example, geophones, may be positioned at various locations to receive the reflected and refracted sound or acoustic waves and convert them into corresponding electrical signals. The signals produced by such seismic sensors are then analyzed for the presence and extent of formations containing oil and gas deposits.
Geophone-type sensors conventionally include a spring mounted electrical coil which is positioned within the field of a permanent magnet. The geophone is conventionally clamped, or otherwise fixed, to a solid structure which transmits seismic waves to the geophone. For example, geophones may be staked to the surface of a terrain which is being surveyed. Seismic waves may then be transmitted from a seismic source, through the various subterranean features which then reflect and refract the waves, which then travel through the stakes, and into permanent magnet of the geophones. The permanent magnet is displaced and may oscillates while the electrical coil floats in a given axis relative to the permanent magnet, and thus relative to the magnetic field, thereby inducing an electrical change through the coils responsive to the relative displacement between the magnet and coils. This electrical change is recorded as a signal representative of the seismic waves.
It is noted that geophone-type sensors are conventionally one-dimensional detecting and recording devices. This is a result of the geophone's basic design, wherein the coil is displaced along a defined axis relative to the permanent magnet. Thus, in order to properly detect and record seismic activity within a given formation, multiple geophone sensors, oriented at orthogonal axes relative to one another, may be employed. Indeed, geophones, or geophone modules, may include three separate sensors, with the sensors being respectively oriented, for example, in along the X, Y and Z axes of a Cartesian coordinate system.
More recently, geophones have been employed in downhole environments in an effort to improve the accuracy of seismic surveys. However, in doing so, such geophones still have to be clamped to a fixed structure, such as, for example, by pressing the geophone firmly against the side wall of the well bore, in order to detect any seismic waves which may be transmitted therethrough. The use of clamping mechanisms requires that additional components to be deployed downhole, additional controls be implemented within a given surveying system and generally increases the complexity of a given surveying operation. Furthermore, geophones are often arranged as strings or other longitudinally extending structures where multiple geophones are spaced apart significant distances in order to obtain seismic data at multiple locations within a well bore. The length of such assemblies presents additional complexity in clamping the geophones within the well bore, as clamping must occur at multiple locations along the assembly.
Additionally, even with adequate clamping mechanisms, it can be difficult to effectively couple the geophone with a specified fixed structure, such as the side wall of a well bore, in a downhole environment. For example, often times there is a coating of built-up drilling fluid or “mud” or other material on the surface of a well bore wall. Thus, in such cases the “clamping” of the geophone is with the built-up layer of material which does not effectively transfer seismic waves present in the surrounding formation. Instead, the “clamping” of a geophone with the built-up layer of material may in fact lead to the detection of seismic data which is incomplete and/or incorrect.
In view of the shortcomings in the art, it would be advantageous to provide a method and apparatus which allows for accurate and effective detection of acoustic waves in a downhole environment, and which eliminates the need for mechanical coupling with a fixed structure thereof. It would further be advantageous if such a method and apparatus were compatible with conventional seismic surveying techniques and processes.